A=[ ...
20, -20,  0,   0,  0,   0; ...
 0,   0,  3, -12,  0,   0; ...
 0,   0,  0,   0,  2,  -3; ...
 1,   1,  0,   0,  0,   0; ...
 0,   0,  1,   1,  0,   0; ...
 0,   0,  0,   0,  1,   1; ...
 ]
B= [ 0; 0; 0; 2; 1; 3 ]
% 1st method
x=inv(A)*B
% 2nd method
x=A\B


